Electrical Demonstration Equipment

Abstract

IN the Bell Laboratories Record of July a description is given by C. D. Hanscom of a variety of equipment which has recently been assembled by the Bell System for use in public lectures. One of the most unusual demonstrations is a Rochelle salt crystal which flashes a neon lamp when hit with a gavel. This illustrates how a change in mechanical dimensions caused by the blow generates momentary voltages of considerable magnitude by the piezoelectric effect. A bar of steel (a permanent magnet) floating in mid–air demonstrates the power of modern magnets. A permanent magnet concealed in the base of the apparatus repels the bar, holding it up against the force of gravity; a full packet of cigarettes can be supported in addition to the bar. There are also in the collection permalloy rods which are so permeable that they are magnetized by the earth's field, when held pointing north at or near the angle of declination. This is demonstrated by their ability to attract and hold short pieces of permalloy tape.